1. Field of the Invention
The present invention relates to a clock generator, and more particularly, to an apparatus and method for generating a clock using piecewise linear modulation.
2. Description of the Related Art
Due to increasing use and demand for various portable devices, electromagnetic interference (EMI) between components of a portable device is becoming an increasingly large problem. A simple EMI problem may be transient distortion of video or audio data, but a serious EMI problem may cause components to malfunction. With miniaturization and integration of portable devices, methods of coping with EMI are under active discussion. Products are predicted to be further integrated and miniaturized and to operate at higher speeds in the future, and thus demand for technology for preventing EMI will further increase.
To prevent on-chip EMI, components of a portable device may be covered. However, since the size and cost of the device drastically increases, this method is not feasible.
A method of increasing rising time to reduce high-frequency components of a signal can be simply implemented using Resistive Capacitive (RC) delay when a portable device has a clock frequency of about 30 MHz or less. However, when a delay is shortened due to increase in the clock frequency, it is difficult to adjust skew and control rising time. Thus, it is impossible to efficiently reduce radiated electromagnetic waves by increasing rising time in a system having a high clock frequency.
Currently, spread spectrum technology is mentioned as the most effective method for reducing EMI.
Spread spectrum clocks are compatible with a conventional clock driving method and can use an extended frequency of several hundred MHz to several GHz. In addition, since a clock signal is modulated, a signal applied to all circuits synchronized with the clock to operate is also modulated. When the clock is modulated, not only all data but also noise generated on a power line are modulated. In this way, spread spectrum clocks reduce EMI generated in digital systems.
The spread spectrum technology modulates a period or frequency of a regular clock to evenly distribute energy concentrated on a specific frequency band over a wider frequency band.
Most conventional spread spectrum clock generators introduce an intended jitter into a regular clock using a Phase-Locked Loop (PLL) or Delay Locked Loop (DLL) to generate a spread spectrum clock. However, such a jitter deteriorates the performance of an analog front end of a microprocessor, and a relatively complex circuit is necessary to demodulate the spread spectrum clock into the regular clock.
Therefore, a method of inverting the phase of a regular clock according to a specific modulation profile to generate a spread spectrum clock has been developed.
As illustrated in FIG. 1A, a conventional spread spectrum clock generator 100 using phase inversion roughly includes a multiplexer 106, an inverter 108, a pattern generator 114 and a divider 126.
When a regular clock 102 is input through an input terminal 104 of the spread spectrum clock generator 100 using phase inversion, it is uniformly applied to the multiplexer 106 and the inverter 108. The inverter 108 applies a clock 110 having an inverse phase with respect to the regular clock 102 to an input terminal 112 of the multiplexer 106. The pattern generator 114 also receives the regular clock 102 and outputs a modulation signal 116, and a modulation profile is determined according to a constitution of the pattern generator 114. The multiplexer 106 selects one of the regular clock 102 and the clock 110 having the inverse phase according to the modulation signal 116, and outputs a spread spectrum clock 124 having a modulated phase. In addition, the phase-modulated spread spectrum clock 214 may be demodulated by the divider 126 into a regular clock 128 and used in an analog front end of a microprocessor.
There are two modulation schemes frequently used in Spread-Spectrum Clock Generators (SSCGs). A triangular modulation scheme of FIG. 1B outputs a linear signal and thus is readily implemented on a chip.
A Hershey-Kiss modulation scheme of FIG. 1C reduces peaks shown at both ends of an output clock based on the triangular modulation scheme to maximize reduction in EMI.
However, when a conventional clock generator uses the triangular modulation scheme, peaks are shown at both ends of an output clock, and thus reduction in EMI decreases. Since the Hershey-Kiss modulation scheme outputs a non-linear signal, it is difficult to implement on a chip and thus is mostly implemented using a memory.